1. Field of the Invention
The present invention relates to digital imaging systems and methods, and more particularly to digital imaging systems that apply image information along helical paths.
2. Description of the Related Art
In many digital graphic-arts applications, recording media are imaged by arrays of imaging devices. For example, to circumvent the cumbersome photographic development, plate-mounting and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate or recording medium. Plate-imaging devices amenable to computer control include various forms of lasers. For example, U.S. Pat. Nos. 5,351,617 and 5,385,092 disclose ablative recording systems that use low-power laser discharges to remove, in an imagewise pattern, one or more layers of a lithographic printing blank, thereby creating a ready-to-ink printing member without the need for photographic development. In accordance with those systems, laser output is guided from the diode to the printing surface and focused onto that surface (or, desirably, onto the layer most susceptible to laser ablation, which will generally lie beneath the surface layer). Other systems use laser energy to cause transfer of material from a donor to an acceptor sheet, to record non-ablatively, or as a pointwise alternative to overall exposure through a photomask or negative.
In such systems, the imaging devices are spaced apart by a distance (the “device pitch”) much greater than the resolution—i.e., the distance between image dots (the “dot pitch”) as they appear on the recording medium. The devices are located adjacent to a cylinder, and as the cylinder rotates, the devices each image a series of circumferential rings. That is, each device images through a full rotation of the cylinder, producing a line of the image on the recording medium. The devices are then advanced axially, i.e., indexed, one unit of resolution along the cylinder to image the next circumferential line. Typically the recording medium is in the form of a sheet pinned to the cylinder; the circumferential region along the cylinder between the ends of the recording medium is a gap or “void” segment, and it is when the devices overlie this area that they are advanced.
An alternate mode of imaging follows a spiral or helical pattern. Instead of a series of sequential rings, an imaging device traces a continuous helix over the plate cylinder. In one known arrangement, each device in a linear array is advanced one unit of resolution (i.e., the dot pitch) during each rotation of the cylinder. The devices are each responsible for imaging an adjacent zone, so the end of the helical path followed by one device coincides with the point at which the next device began its helical path. As a result, the final image is a continuous helix. This approach requires no indexing of the imaging devices, and can image over a continuous cylindrical recording medium without a void. Unfortunately, because the zones are adjacent, this type of system produces visible artifacts arising from differences in the imaging characteristics of adjacent devices.